Most likely because they are the cells from which all specialised cells Stem, i.e. stem cells are the precursors which form more differentiated types of cells, e.g. blood, bone are formed from, or...
egnore that... a stem and left plot, it is called that because a leaf is on a stem just like in the stem and leaf plot work, the stem is the first number... example stem =1 on 12 and the leaf is 2....